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You bring up a great question with regard to WiFi calling on the Galaxy S8! Since this is a non-T-Mobile device, we cannot guarantee that WiFi calling will work. You can read more about it here:

Non T-Mobile Wi-Fi Calling-capable devices

Wi-Fi Calling is available on non-T-Mobile iPhone 5c, 5s, 6, 6 Plus and newer on iOS 8 or later. When a T-Mobile SIM card is inserted into these iPhones, the device receives the T-Mobile Carrier Bundle and Wi-Fi calling options become available.

Not available on iPhone 5 or older devices, even with iOS 8 or later.

Not available on other non T-Mobile devices.

Some manufacturers state their devices have T-Mobile Wi-Fi Calling (for example, Nexus 5x and 6p). T-Mobile doesn't sell them and hasn't tested them for compatibility, so we can't verify compatibility with these devices.